The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for improving signal-to-noise ratio in using a local, per-voxel estimation of noise covariance.
Parallel imaging with receive coil arrays provides many advantages for MRI, including increased image signal-to-noise ratio (“SNR”) and the ability to reduce scan times using accelerated parallel imaging. Together, these advantages can increase the time efficiency of an MRI examination or enable new imaging techniques afforded by the increased sensitivity or speed of the acquisition.
When receiving the MRI data in a set of receiver coils in parallel, the data from each independent channel must be combined to form a single image. There are several approaches to this combination, and with extra information about the coupling of the data across the array a combination can be performed that maximizes the final SNR. Typically this coupling is estimated with a pre-scan that measures the intrinsic coupling of thermal noise across the detectors for the given object being imaged, and this coupling depends both on how the coil detectors couple to the object and on how the detectors couple with one another. This coupling, given by the noise covariance matrix, can then be integrated into the combination for increased SNR and a subsequent increase in imaging performance. The noise covariance across channels, however, can be affected by the accelerated parallel imaging algorithm.
Image reconstruction algorithms used in parallel imaging techniques often combine information across the channels of the coil array and, therefore, affect the noise coupling in the resulting individual channel images to be combined. Also, in some applications the image SNR is not the quantity that should be maximized. For example, in functional MRI, the temporal SNR is the quantity of interest that should be maximized to optimize the ability to detect subtle brain activations in the time-series data. To maximize the temporal SNR, the coil combination must take into account the coupling of the time-series data across the coil array channels. This coupling is affected by the physiological noise and other system instabilities and temporal fluctuations.